1. Field of the Invention
The present invention relates to a fuel cell in which a fuel gas is supplied from a central region to an outer region of an anode, an oxygen-containing gas is supplied along a cathode, and an exhaust gas as mixture of the fuel gas and the oxygen-containing gas after consumption is discharged to the outside from an outer region of an electrolyte electrode assembly. Further, the present invention relates to a method of producing the fuel cell.
2. Description of the Related Art
Solid oxide fuel cells (SOFC) employ an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between a pair of separators (bipolar plates). In use, a predetermined number of the separators and the electrolyte electrode assemblies are stacked together to form a fuel cell stack.
In some of the fuel cells of this type, no seals are provided for the separators, and the fuel cell stack is placed in a container. In the structure, a fuel gas supplied to the anode and an oxygen-containing gas supplied to the cathode are discharged from an outer end of the separator after power generation reaction. Therefore, the fuel gas containing fuel which has not been consumed in the reaction and the air are mixed together to induce combustion around the outer region of the separator. The mixed gas is discharged to the outside of the container as an exhaust gas.
During power generation, the flow rate of the supplied air is larger than the flow rate of the supplied fuel gas. Therefore, the remaining oxygen is present in the exhaust gas. Further, the fuel gas containing fuel which has not been consumed in the reaction is present at the anode, and water vapor is generated at the anode due to the power generation reaction. Therefore, the end surface of the separator facing the anode is exposed to the reducing atmosphere while being exposed to the oxygen flowing around to the anode from the cathode and the water vapor. That is, a reducing atmosphere, an oxidizing atmosphere, and a water vapor oxidizing atmosphere are present around the end surface of the separator facing the anode.
For example, the anode is made of cermet material of Ni—Zr (nickel-zirconium) or the like. The material in the outer end of the anode is oxidized to NiO. NiO has a high electrical resistance, and prevents electrical conductance. The MEA is damaged easily by the local volume change (expansion or contraction) due to the oxidation/reduction reaction.
Further, in general, the separator is made of iron alloy such as stainless steel. Therefore, Fe oxides or Cr oxides are produced on the surface of the separator due to oxidation or water vapor oxidation. Thus, electrical conductivity of the separator is lowered. Further, problems such as increase in pressure loss in the fuel gas, non-uniform flow of the fuel gas, peeling of oxides, dispersion of peeled oxides into the fuel cell and increase in the contact resistance between the separator and the electrolyte electrode assembly occur.
In a proposed technique disclosed in Japanese Laid-Open Patent Publication No. 05-036425, a nickel plating layer is provided on a fuel-electrode-side surface of the separator of heat resistant alloy by a wet plating process to prevent oxidation of elements in the heat resistant alloy, and to prevent increase in the electrical resistance.
Further, in a technique disclosed in Japanese Laid-Open Patent Publication No. 2006-236600, ferrite stainless steel containing aluminum is adopted as material of a separator to prevent water vapor oxidation of the separator. An alumina film is formed on a surface of the separator using the aluminum as a source.
Further, techniques of forming an oxide film on the surface a metal separator is disclosed in Japanese Laid-Open Patent Publication No. 2002-289215 and Japanese Laid-Open Patent Publication No. 2005-327499. The oxide film is derived from the metal separator. Further, silver plating is provided on the oxide film.
In the above techniques, a film or a layer is formed on the separator. In another proposed technique, a protection plate is interposed between the metal separator and the anode (e.g., see Japanese Laid-Open Patent Publication No. 2005-259684).
Further, Japanese Laid-Open Patent Publication No. 09-115525 discloses a technique of forming electroless plating on a fuel electrode comprising cermet of nickel and material chiefly containing nickel and zirconia, and forming a nickel plating film on a surface of the fuel electrode to reduce the contact resistance at contact points for improving power generation efficiency.
In the conventional technique disclosed in Japanese Laid-Open Patent Publication No. 05-036425, the nickel plating layer undesirably undergoes water vapor oxidation to become a nickel oxide layer. The nickel oxide is a p-type semiconductor. Since electrons do not move actively, if a large amount of nickel oxide is present, the power generation performance of the fuel cell may be degraded undesirably.
Further, in the conventional technique of forming a known alumina film as an insulator as disclosed in Japanese Laid-Open Patent Publication No. 2006-236600, the internal resistance of the fuel cell increases, and the current collecting performance of the separator is lowered undesirably.
Further, in the conventional technique of performing silver plating as disclosed in Japanese Laid-Open Patent Publication No. 2002-289215 and Japanese Laid-Open Patent Publication No. 2005-327499, and in the conventional technique of performing nickel plating as disclosed in Japanese Laid-Open Patent Publication No. 09-115525, the cost required for the plating process is high.
Further, in the conventional technique disclosed in Japanese Laid-Open Patent Publication No. 2005-259684, since the protection plate is interposed between the separator and the anode, the thickness in the stacking direction of the fuel cell becomes large disadvantageously.
Moreover, in any of the above conventional techniques, anti-corrosion property of the separator in the environment where the three atmospheres, i.e., the reducing atmosphere, the oxidizing atmosphere, and the water vapor oxidizing atmosphere are present is not taken into consideration. Further, by the oxide film generated on the surface of the separator, current collection performance of the separator is lowered, the fuel electrode (anode) and the separator do not contact each other sufficiently tightly, and increase in the contact resistance occur disadvantageously. However, no solutions for the problems or disadvantages have been proposed.